The present invention relates to an antenna apparatus and more particularly to an antenna apparatus which is mounted, for example, in the interior of a motor vehicle, and enables good sensitivity to be secured.
A typical on-board antenna apparatus is mounted in any position inside a vehicle, preferably mounted near the window of the vehicle. The antenna apparatus is installed with a helical antenna element in the form of a helical conductor, for example. The helical antenna element is an omnidirectional antenna element as shown by a radiation pattern in FIG. 11, for example. The mounting of the antenna element is carried out by attaching the base of an antenna case securely onto a portion inside a motor vehicle normally with a double-sided tape, an adhesive, machine screws and so forth. (refer to JP-A-2003-37430)
Such an on-board antenna apparatus is adapted to receive various signals including global positioning system (GPS) signals, satellite radio broadcasting signals, cellular telephone signals and the like.
As described above, the on-board antenna apparatus employing the helical antenna element is usually an omnidirectional antenna. However, regarding the directional sensitivity of the antenna apparatus, it is difficult to secure optimum sensitivity when the antenna apparatus is actually installed because the antenna apparatus is mounted inside the vehicle but not set in such an environment that a shielding member is existent. Moreover, since the mounting of the antenna apparatus is carried out by attaching the base of the antenna case securely onto any planar face in the interior of the vehicle as described above, the antenna apparatus for use cannot be tilted in the direction toward a satellite after the antenna apparatus is installed in the interior of the vehicle.